Fix null reference error during reflow

- Issue MatterHackers/MCCentral#2140
Investigate AddToQueueFromLibraryButtonAddsItemsToQueue

Library/Widgets/ListView/IconListView.cs

@@ -132,6 +132,8 @@ namespace MatterHackers.MatterControl.CustomWidgets
 
 		public ListViewItemBase AddItem(ListViewItem item)
 		{
+			reflowingContent = true;
+
 			var iconView = new IconViewItem(item, this.ThumbWidth, this.ThumbHeight);
 			iconView.Margin = new BorderDouble(leftRightMargin, 0);
 
@@ -139,6 +141,8 @@ namespace MatterHackers.MatterControl.CustomWidgets
 
 			AddColumnAndChild(iconView);
 
+			reflowingContent = false;
+
 			return iconView;
 		}